In a radio communication system, such as a radio telecommunications system having base stations and large numbers of mobile stations which may be in communication with each base station, it is necessary to control the power with which signals are transmitted. For example, it is advantageous to reduce transmit power as far as practicable, but the transmit power must be high enough to provide a required signal level at the receiver. In the case of a direct sequence code division multiple access (DS-CDMA) system, the signals transmitted from the different mobile stations should preferably all be received at the base station at the same level.
Power control is performed by measuring the signal-to-interference ratio (SIR), and controlling the transmit power so that this reaches a target value.
For example, U.S. Pat. No. 5,773,030 describes a spread spectrum communication system, in which a base station sends an adjustment signal to a mobile station, to control the power of transmissions from the mobile station. The adjustment signal is calculated to increase the power level if the received power level is below a threshold, and to decrease the transmit power level if the received power level is above a threshold. The threshold is set at a value which depends on the measured speed of the mobile station.
The paper “SIR-Based Transmit Power Control of Reverse Link for Coherent DS-CDMA Mobile Radio”, by Seo, et al, IEICE Trans. Commun. Vol.E81-B, No. 7 July 1998 pp1508-1516, describes an alternative system. In that system, the transmit power of a mobile station is controlled in response to a TPC (transmit power control) signal sent from a base station. The TPC is calculated on the basis of a comparison performed in the base station between, on the one hand, an estimate of the signal-to-interference (SIR) ratio of signals received from the mobile station at the base station and, on the other hand, a target signal-to-interference value. The speed of movement of the mobile station is mentioned as a parameter which has an effect on the optimum size of the steps in which the TPC signal can be controlled.